A transmission unit of a communication device, such as a cellular phone or a wireless LAN, is required to operate with low power consumption. Such a transmission unit of a communication device is required to operate with low power consumption regardless of the magnitude of output power, and is also required to ensure a high accuracy of transmission signals. In particular, a power amplifier disposed at a final stage of a transmission unit of a communication device occupies 50% or more of the power consumption of the entire transmitter. For this reason, the power amplifier disposed at the final stage of the transmission unit of the communication device needs to have a high power efficiency.
The power amplifier typically includes one transistor. A radio signal input to a gate is converted into current information by the transistor. The converted information is output to a load from a drain terminal through a matching circuit. The drain terminal is connected to a power supply in parallel with the matching circuit through an inductor. The efficiency ηd of the power amplifier is in proportion to envelope components of the radio signal. The efficiency ηd of the power amplifier becomes the maximum efficiency ηdmax when an envelope voltage in the drain terminal is equal to a power supply voltage. When the envelope voltage becomes larger than this, this produces significant signal distortion, thereby causing a failure of communication. Accordingly, the envelope voltage is typically adjusted so that the peak value of the envelope voltage of the radio signal becomes equal to the power supply voltage. This means that an amplitude value of the signal changes in time, and as the rate (backoff) of the average amplitude value to the instantaneous maximum amplitude value increases, the average efficiency decreases by the amount corresponding to this rate. For example, with a signal having a backoff of about 7 dB (2.24 times in the amplitude ratio), the efficiency ηd of the power amplifier is about 1/2.24 of ηdmax.
In recent years, an envelope tracking system has been attracting attention as a system to improve the average power efficiency of a power amplifier. In this system, a power supply terminal of the power amplifier is connected to a power supply modulator having a variable voltage value through an inductor. A voltage output from the power supply modulator is controlled to follow envelope components of the radio signal output from the power amplifier. In this system, a voltage applied to the envelope voltage and a voltage applied to the power supply terminal of the power amplifier through the inductor are always equal to each other. Thus, the power amplifier always operates with the maximum efficiency ηdmax.
When the power efficiency of the power supply modulator is represented by ηv and the power efficiency of the power amplifier is represented by ηd, the power efficiency ηa of the whole system in the envelope tracking system can be expressed by the product of ηv and ηd, as shown in the following expression (1).ηa=ηd·ηv  (1)
The power efficiency ηd of the power amplifier is fixed to ηdmax when the output voltage of the power supply modulator ideally matches an envelope signal. Accordingly, the power efficiency of the whole system is in proportion to the power efficiency ηv of the power supply modulator.
FIG. 8 is a block diagram showing a configuration of a power supply modulator 200 according to a typical envelope tracking system (Non-patent literature 1). The power supply modulator 200 includes a variable voltage source 21, a linear amplifier 22, a current sensor 23, and an inductor 25. In the power supply modulator 200, a radio signal WS is input to a power amplifier 24 connected to the power supply modulator 200, and at the same time, an envelope signal ES of the radio signal WS is input to a control terminal of the linear amplifier 22. The linear amplifier 22 outputs a current so that an output voltage becomes equal to a voltage signal input to the control terminal. The current sensor 23 includes an amplifier 231 and a resistor 232. Each end of the resistor 232 is connected to an input terminal of the amplifier 231. The current sensor 23 determines whether an output current of the linear amplifier 22 is a source direction or a sink direction to output a voltage control signal to the variable voltage source 21. The variable voltage source 21 is connected to the power amplifier 24 through inductors 25 and 26.
When the output current of the linear amplifier 22 is the source direction, the voltage value of the variable voltage source 21 is set to a high value. Thus, the value of the current supplied from the variable voltage source 21 to the power amplifier 24 increases. Further, the current output from the linear amplifier 22 in the source direction decreases by the amount corresponding to the increase in the current supplied from the variable voltage source 21. Meanwhile, when the current of the linear amplifier 22 is the sink direction, the voltage value of the variable voltage source 21 is set to a low value. Thus, the current output from the variable voltage source 21 decreases. The current in the sink direction of the linear amplifier 22 decreases by the amount corresponding to the decrease in the current output from the variable voltage source 21. The above operation corresponds to an operation of correcting a difference between a current supplied from the variable voltage source 21 and a current required to reproduce envelope signals ES by the linear amplifier 22.
Assume herein that the efficiency of the variable voltage source 21 is 100%. Further, power consumed by the variable voltage source 21 is represented by Pvvar, power consumed by a power supply of the linear amplifier 22 is represented by Pla, and heat loss generated in the linear amplifier 22 is represented by Plaloss. In this case, the efficiency ηv of the power supply modulator 200 is expressed by the following expression (2).ηv=(Pvvar−Plaloss)/(Pvvar+Pla)  (2)
When it is possible to reproduce envelope signals only by the current of the variable voltage source 21, the linear amplifier 22 does not operate. Since the power consumption Pla and the heat loss Plaloss become zero, the power efficiency ηv of the power supply modulator 200 is 100%. Accordingly, from the expression (1), the efficiency ηa of the whole system becomes equal to the theoretical maximum efficiency ηdmax of ηd.